It is known that the frequency of an oscillator can be locked to the frequency of variations in the intensity of light in an optical signal injected into the active circuit by either direct or indirect illumination of the active element. For any given amount of modulated light intensity, there is a given bandwidth of frequencies (.DELTA.f), called the locking bandwidth, to which the oscillator can be locked. The phase and frequency coherency of these modules can be efficiently obtained by subharmonic injection locking of the local oscillators (LO) via a fiber optic link. The local oscillator can be parametrically stabilized to a reference signal using the nonlinearity of both optical (lasers, electro-optic modulators) and electronic devices (such as HEMT, FET, HBT) in a fiber optic fed phased array architecture. More specifically, laser and FET nonlinearities create harmonics of the reference signal close to the oscillation frequency of the local oscillator, which results in the fundamental injection locking of the local oscillator and which also results in the subharmonic injection locking with respect to the synchronizing frequency reference.
As those skilled in the art will know, the locking range and noise behavior of subharmonic injection locked local oscillators are highly dependant on the oscillator device nonlinear behavior and the parameters of the feed back network. Therefore, even though the injection locked oscillator will provide frequency synchronization, the initial frequency detuning between the free running oscillation and the synchronizing signal will cause an unwanted phase shift of +/-.pi./2 over the locking range.
Using a directly modulated laser diode, FET oscillator locking has been demonstrated at frequencies up to 15 GHz. See Higgins, "Direct Optical Subharmonically Injection Locked MESFET Oscillators," Microwave and Optical Technology Letters, Vol. 6, No. 1, January, 1993. Higher locked frequencies are possible, however, the narrow locking range attainable render the method impractical. See Mizuno, "Microwave Characteristics of an Optically Controlled GaAs MESFET," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-31, pp. 596-600, July 1983; de Salles, "Optical Control of GaAs MESFETS," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-31, pp. 812-820, October 1983; and Gautier et al, "Optical Effects of the Static and Dynamic Characteristics of GaAs MESFET," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-33, pp. 819-822, September 1985, respectively. Also see Esman et al, "Optical Phase Control of an Optically Injection-Locked FET Microwave Oscillator," IEEE Transactions on Microwave Theory and Techniques, Vol. 37, No. 10, October 1989.
These optically injection-locked oscillators are attractive prospects for airborne and space communications, radar, imaging and surveillance applications since the locking signal can be distributed using light weight optical fiber. The growing availability of wide-bandwidth laser diodes has further stimulated interest in this injection locking technique. However, the potential applications for this injection locking technique, such as phased array radar and microwave power combining, require not only frequency-locked (coherent) sources but also individual source phase control. Thus, optical techniques for phase control have been generally limited to optically controlled varactors and to fiber stretching and fiber length switching techniques. These techniques suffer from a number of drawbacks, including limited phase shifts, discrete phase shifts, slow response, moderate to prohibitive insertion loss, high voltage requirements, and small phase modulation bandwidths, as well as the size and weight of the additional components required.
Therefore, there exists a need to provide for a synchronized optically controlled microwave/millimeter wave oscillator which serves as a single component for the front end of a microwave transmission system that can act as a stable oscillator, mixer and a phase shifter. The present invention fulfills these needs.